1. Field of the Invention
The present invention relates to an apparatus for manufacturing large-diameter silicon single crystals by the Czochralski method (hereinafter referred to as a CZ method).
2. Description of the Prior Art
The silicon single crystals used in the field of LSIs have been generally manufactured the CZ method and in accordance with this CZ method the amount of molten silicon within the crucible is decreased with the growth of a silicon single crystal. Therefore, as the silicon single crystal grows, the dopant concentration in the silicon single crystal is increased and the oxygen concentration is decreased. In other words, the properties of the silicon single crystal varies along the direction of its growth.
Since the quality required for silicon single crystals has been becoming increasingly severe year after year along with the tendency toward increasing the level of integration of LSI's, problem has been caused that the yield is decreased correspondingly.
As a means of overcoming this problem, as for example, Japanese Patent Publication No. 40-10184 (P1, Right column, L20 to L35) discloses a method (the continuous CZ method) in which the interior of a quartz crucible of the CZ method is divided by a cylindrical or crucible-type quartz partition member having at least one small hole and a silicon single crystal is grown on the inner side (the single crystal growing section) of the partition member while feeding starting material silicon to the outer side (the material melting section) of the partition member.
As also pointed out by Japanese Laid-Open Patent No. 62-241889 (Page 2, Right Upper Column, L2 to L160), this method has a serious disadvantage that the molten silicon tends to be solidified starting at the partition member on the inner side of the partition member.
In other words, the solidification is caused at the portion where the surface of the molten silicon in the single crystal growing section is in contact with the partition member. This solidification grows toward the crucible central portion where the temperature is low thereby impeding the growth of the silicon single crystal. This is due to the following cases.
Since silica glass usually used as the partition member tends to pass heat radiation and since moreover there is a large heat radiation to the water-cooled furnace wall from the portion of the upper part of the partition member which is exposed above the surface of the molten silicon in the usual case, the heat in the molten silicon is transmitted upwardly through the partition member and the heat is radiated from the portion of the partition member which is exposed above the molten silicon surface. As a result, the molten silicon temperature is greatly decreased in the vicinity of the partition member.
In this way, the molten silicon surface contacting with the partition member is in a condition having an extremely high tendency toward causing the occurrence of solidification.
Recently, the manufacture of high-quality granular silicon has become possible and it is considered to be relatively easy to feed such granular silicon continuously into the molten silicon as the starting material silicon in the continuous CZ method.
However, when feeding the granular silicon onto the molten silicon surface, if a sufficient fusing heat is not imparted to the granular silicon, a part of the granular silicon is caused to remain unmelted. Then, it is not infrequent that solidification is caused from the unmelted part of the granular silicon and the solidification is spread.
This is caused by the fact that due to the difference in density, the solid granular silicon floats on the molten silicon surface, so that, since the emissivity of the solid granular silicon is greater than that of the molten silicon, the heat tends to be lost easily. Particularly, in case the granular silicon is attached and agglomerated to the partition member at the molten silicon surface in the material melting section, as in the case of the solidification in the single crystal growing section, the heat is rapidly lost through the partition member thereby tending to cause the occurrence and spreading of solidification.
In the future, if the amount of start in material supplied is increased along with increase in the diameter of silicon single crystals and increase in the pulling rate, this phenomenon tends to occur more frequently.
It is to be noted that this problem remains unchanged essentially even if the starting material silicon supplied is in any other form than the granular form.
Laid-Open Patent No. 1-153589 proposes a method which prevents the occurrence of solidification at the partition member and which prevents any part of the starting material silicon supplied from being left unmelted. This publication discloses that the partition member and the material melting section are covered with a heat keeping cover so that the radiation of heat to the water cooled furnace wall, etc., above the crucible is prevented and the temperature of the molten silicon around the partition member and in the material melting section is maintained.
Also, with a silicon single crystal manufacturing apparatus based on the conventional CZ method and having no partition member, etc., it is possible to increase the pull rate by using a cover of a shape different from the above-mentioned one.
While a variety of materials such as graphite and metals may be considered as suitable materials for the heat keeping cover, the studies by the inventors, have shown that graphite used generally as the material for the furnace components of the silicon single crystal furnace is high in emissity and therefore its heat keeping effect is not satisfactory. On the other hand, metals which are low in emissity are capable of efficiently preventing the radiation of heat so that they have high heat keeping effect and are well suited for the intended application of the heat keeping cover.
By constructing this heat keeping cover with a metal sheet, it is possible to ensure a stable pulling operation without causing the occurrence of solidification at the partition member and any unmelted part of the supplied starting material in the material melting section even in cases where large diameter silicon single crystals are pulled at high rates.
However, the heat keeping cover is intended for use in the high temperature environment within the single crystal pulling furnace and therefore it is necessary to be made of a high melting point metal such as tantalum, molybdenum or tungsten. Especially, tantalum is a metal which is excellent in malleability so that it can be easily worked in various ways and it is easy to use.
By thus covering the partition member with the heat keeping cover made of a material selected from the group consisting of a tantalum sheet, molybdenum sheet and tungsten sheet, it is now possible to prevent the molten silicon from solidifying at the partition member and prevent any part of the supplied starting material to be left unmelted in the material melting section, thereby making it stably grow a silicon single crystal according to the continuous CZ method.
However, it has been found out that the single crystal grown by using this metal-sheet heat keeping cover has a tendency toward increasing the density of the oxidation induced stacking fault (hereinafter referred to as an OSF) to about 10.sup.3 defects/cm.sup.2, so that there is a problem from the crystal quality point of view.